1. Field of the Invention
The present invention relates to a method for reforming Al alloy castings, especially a method for reforming mechanical characteristics of precipitation hardening type Al alloy castings by heating and pressing the Al alloy castings. More particularly, the present invention is concerned with an improved method comprising an appropriate combination of a reforming method using the pressure of a high temperature, high pressure gas (hereinafter referred to as “HIP method”), with heat treatment under atmospheric pressure (solution treatment, quenching, and aging) and which can thereby reform mechanical characteristics of castings efficiently and economically.
2. Description of the Related Art
There is known an HIP method wherein after casting Al alloy, the Al alloy casting is treated in a high temperature, high pressure gas atmosphere to crush pore defects. In the case of an Al alloy which requires heat treatment for ensuring a required strength, HIP treatment is usually followed by re-heating and subsequent solution treatment, water quenching, and aging to ensure a strength characteristic of a target level.
For example, FIG. 1 illustrates known temperature and pressure operation conditions in case of adopting the HIP method. General HIP conditions in case of treating Al alloy castings involve a temperature of 500° to 530° C., a pressure of about 100 MPa, and a treatment time of about 1 to 3 hours. In this case, the time required after loading the workpiece into an HIP apparatus until taking out the workpiece from the apparatus involves the time taken for evacuation and gas purging in the HIP apparatus after the loading of the workpiece and the time taken for heating and pressing and for reducing the temperature and pressure before and after maintaining predetermined high temperature and high pressure, thus requiring an extra time of about 4 hours relative to the actual high temperature/high pressure holding time. As a whole, the required time is about 6 to 8 hours.
In the conventional equipment, a heat treatment apparatus and an HIP apparatus are often located away from each other, so the workpiece after HIP treatment is once allowed to stand in air, then conveyed up to the place where the heat treatment apparatus is installed, and are thereafter re-heated. As shown in FIG. 1, the heat treatment performed after HIP treatment is usually “T6 treatment” which comprises three steps of solution treatment (6 to 10 hours)→water quenching→aging (8 to 12 hours), requiring a total of 21 to 30 hours.
In subjecting an alloy casting which requires such a heat treatment and HIP treatment, the HIP treatment temperature is usually almost equal to or a little lower than the solution treatment temperature, so if it is possible to carry out solution treatment concurrently with HIP treatment, it is considered possible to not only simplify the treatment process but also shorten the required time. Studies have long been made on this regard. Actually, however, due to various problems resulting from the use of a high pressure gas, the technique for carrying out solution treatment concurrently with HIP treatment has not been practically applied yet.
According to another conventional technique, solution treatment is performed concurrently with HIP treatment while changing temperature and pressure as in FIG. 2 (Metallurgical Science and Technology, Vol. 19, No. 1, June 2001, FIG. 6-b). However, due to a problem in operation, temperature is also dropped rapidly at the same time as reduction of pressure after the end of HIP treatment.